Association between SULT1A1 Arg213His (rs9282861) Polymorphism and Risk of Breast Cancer: a Systematic Review and Meta-Analysis

Background: The Arg213His (rs9282861) polymorphism of Sulfotransferase Family 1A Member 1 (SULT1A1) gene has been reported to be associated with risk of breast cancer in some epidemiological studies. However, the results of these studies are conflicting and inconclusive. Therefore, this systematic review and meta-analysis was conducted to evaluate the association of SULT1A1 Arg213His (rs9282861) polymorphism with susceptibility to breast cancer. Study design: A systematic review and meta-analysis. Methods: A comprehensive literature search for eligible studies was conducted in PubMed, Elsevier, Science Direct, Scopus and Google Scholar databases up to October 5, 2017. Pooled odds ratios (ORs) with their corresponding 95% confidence intervals (95% CIs) were used to evaluate the strength of the association using fixed effects models and random effects models. Results: A total of 20 relevant case-control studies involving 11,077 cases and 14,798 controls were included in this meta-analysis. Overall, there was a significant association between the SULT1A1 Arg213His (rs9282861) polymorphism and risk of breast cancer in the allele mode (A vs. G: OR=1.117, 95% CI: 1.011, 1.233, P=0.029) and the homozygote model (AA vs. GG: OR=1.288, 95% CI: 1.036, 1.601, P=0.022). Subgroup analysis based on ethnicity suggested SULT1A1 Arg213His (rs9282861) polymorphism had a subtly increased breast cancer risk among Asian population, but not Caucasians. Further, subgroup analyses, significant associations were observed in hospital based group, RFLP-PCR group, and high quality studies subgroups. Conclusions: This meta-analysis suggested that SULT1A1 Arg213His (rs9282861) polymorphism might be associated with breast cancer risk, especially among Asian population. Moreover, the SULT1A1 Arg213His polymorphism is of high clinical relevance by ethnicity and would be a useful marker to identify patients who are at higher risk for breast cancer.

Introduction reast cancer is one of the most common malignancies among women worldwide 1,2 . Currently, more women survive due to earlier diagnosis and better therapy 3,4 . However, the morbidity and mortality of breast cancer have increased in most developing countries 5 . It is well-established that family or personal history of cancer, nulliparous, and history of hormone replacement therapy and different genetic background are most known risk factors for breast cancer 6 . Breast cancer is one of the first multifactorial conditions to be evaluated using molecular genetics techniques 6-8 . It is estimated that 15-30% of hereditary breast cancer cases are due to known highly penetrant genes such as BRCA1 and BRCA2 9 . More than 1,000 different breast cancer-associated mutations in each gene have been documented. However, a few of these mutations could be clearly identified as pathological 9-11 . Genetic variations in SULT1A1 gene are increasingly studied for an increased different cancer risk because of the critical roles in catalyzing the sulfate conjugation of many hormones, neurotransmitters, drugs, and xenobiotic compounds 12 . In particular, SULT1A1 has high activity towards a wide range of substrates including environmental carcinogens such as tobacco. The SULT1A1 gene is located on chromosome 16p12.1-p11.2, spans approximately 5.1 kb and contains nine exons that range in length from 74 to 347 bp 13 . To date, the Arg213His (rs9282861) polymorphism is most widely studied SNP within SULT1A1 gene. The Arg213His B polymorphism is located in exon 7 with G to A transition, at nucleotide 638 (codon 213) leading to an arginine (Arg) to histidine (His) amino acid substitution 14 . Functional studies have reported that the variant A allele (His allele) is associated with lower sulfotransferase activity and thermal stability compared with the wild-type G allele (Arg allele) 15,16 .
So far, there were so many reports about the association of SULT1A1 Arg213His (rs9282861) polymorphism with breast cancer risk [17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35] . On the whole, these studies results were conflicting and inconclusive, probably due to the relatively small size, ethnic background, uncorrected multiple hypothesis testing, and publication bias. It is known that metaanalysis is a statistical procedure for combining data from multiple studies to produce a single estimate on the same outcome of interest with enhanced evaluation. Therefore, current systematic review and meta-analysis was performed from all eligible studies to derive a more precise estimation the association between SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer risk.

Search Strategy
We performed a systematic literature search of electronic databases such as PubMed, Embase, ISI Wed of Knowledge, Google scholar, Scopus, Cochrane Library, Chinese National Knowledge Infrastructure (CNKI) and Chinese Wan Fang databases to identify studies that evaluated the association between SULT1A1 Arg213His (rs9282861) polymorphism and the risk of breast cancer up to October 5, 2017. The search strategy identified all possible studies using combinations of the following keywords: Sulfotransferase Family 1A Member, SULT1A1 gene, G638A, Arg213His, rs9282861, and polymorphism, genetic polymorphism, single nucleotide polymorphism, SNP mutation, and breast, breast cancer, breast malignancy, breast tumor, breast carcinoma, and breast neoplasm. In addition, manual searching was carried out to ensure that no relevant studies were missed in the database search by scanning the review articles, clinical trials, and metaanalyses. No language restrictions were imposed.

Inclusion and Exclusion Criteria
To be eligible for the inclusion criteria in the meta-analysis, the following criteria were used: a) case-control or cohort design; b) evaluating the associations between SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer risk; c) providing sufficient data to estimate the odds ratio (OR) with the corresponding 95% confidence interval (95%CI). Studies were excluded if the following criteria were satisfied: a) Abstracts, case reports, case-only articles, family-based studies, reviews articles and repeated literatures; b) duplicate of previous publications; c) not provided sufficient data (e.g. neither the frequency nor the number of genotype was given) for calculation.

Data Extraction
All data were extracted independently by two authors using a standardized data extraction form and according to the inclusion criteria listed above. The following elements were extracted for each included study: first author name, year of publication, country, ethnicity, source of control groups, genotyping method, total number of cases and controls, minor allele frequency (MAF) in control subjects, P value of the Hardy-Weinberg equilibrium (HWE) test in the control. Different ethnic descents were classified as Caucasian, Asian, Latinos or Mixed (derived from an admixture of different ethnic groups). Disagreements between the two investigators were resolved by discussing with a third investigator. If more than one article were published using the same data, only the newest study and with largest sample size was selected. Moreover, the quality of selected studies was tested by the confirmation of HWE in control groups, and studies without the confirmation of HWE in controls were defined as lowquality studies, while studies with the confirmation of HWE in controls were defined as high-quality studies.

Statistical Analysis
Crude odds ratios (ORs) with corresponding 95% confidence intervals (CIs) were used to assess the strength of association between the SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer risk. The pooled ORs were determined for SULT1A1 Arg213His (rs9282861) polymorphism under the allele model (A vs. G), the homozygote model (AA vs. GG), the heterozygote model (AG vs. GG), the dominant model (AA+AG vs. GG), and the recessive model (AA vs. AG+GG). The significance of the pooled OR was determined by the Z-test, in which P<0.05 was considered as statistically significant. The Chi-square-based Q-test and I 2 -statistics were employed for evaluating the between-study heterogeneity 36,37 . A P-value greater than 0.10 for the Q-test or I 2 less than 50% indicates a lack of heterogeneity among studies, so the pooled OR estimate of the included studies was obtained by the fixed-effects model (Mantel-Haenszel method) 38 . Otherwise, the random-effects model (DerSimonian-Laird method) was used 39 . Hardy-Weinberg equilibrium (HWE) in the control group was calculated by the Chi-square test for goodness of fit, If P value > 0.05, the genotype distribution of control population conformed to HWE. Furthermore, to explore the sources of heterogeneity, subgroup analysis we conducted based on ethnicity, source of controls and studies quality. To validate the reliability of the results, sensitivity analysis was performed though omitting one case-control study each time, as well as limiting this meta-analysis to studies which were conformed to HWE. Publication bias was evaluated by Begg's test and Egger's test (P <0.05 was considered statistically significant) 40,41 . If publication bias existed, the Duval and Tweedie non-parametric ''trim and fill'' method was used to adjust for it. All statistical analyses including meta-regression, sensitivity analysis, subgroup analysis, and publication bias assessment were performed using Comprehensive Meta-Analysis (CMA) software version 2.0 (Biostat, USA). Twosided P values < 0.05 were considered statistically significant.

Characteristics of Studies
As shown in Figure 1, we initially identified 187 potentially relevant studies from database searching and 3 studies from manual retrieval. After screening of titles or abstracts duplicates were excluded resulting in 112 publications. An additional 92 publications were excluded because the studies were reviews, case reports, letter to editor, not reporting the usable data, other polymorphisms of SULT1A1 gene, not related to breast cancer or reported other diseases. Finally, a total of 20 studies with 11,077 cancer cases and 14,798 controls met the inclusion criteria. Study characteristics are summarized in Table 1. The included studies were published from 2000 to 2013. Of those studies, there were twelve studies of Caucasian descendants and eight studies of Asian descendants. The countries of these studies were USA, Austria, India, Korea, Sweden, Germany, Finland, China, Taiwan, Russia, and Italy. All the 20 eligible studies were case-control studies, 11 of them were in a populationbased design and 8 studies were hospital based. The genotyping for SULT1A1 Arg213His (rs9282861) polymorphism was performed by five genotyping methods including real-time PCR (RT-PCR), PCR-restriction fragment length polymorphism (PCR-RFLP), TaqMan, Genotyping by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) and direct Sequencing. The distribution of genotype in the controls of the studies was not in agreement with Hardy-Weinberg equilibrium in six studies, which were further tested in the sensitivity analyses and subgroup analysis of studies with high quality. The detailed characteristics of each study and SULT1A1 Arg213His (rs9282861) polymorphism genotype distributions included in the meta-analysis are presented in Table 1.   Table 2).

Minor allele frequencies (MAFs)
The MAFs for the SULT1A1 Arg213His (rs9282861) polymorphism in the healthy controls is presented in Table 1. The allele and genotype distributions of SULT1A1 Arg213His (rs9282861) polymorphism exhibited ethnic variations. The allele and genotype distributions of 213His allele in the Caucasian and Asians populations were 32.75% (17.5%-48.0%) and 7.4% (2.1%-12.7%), respectively. Thus, the frequency of the 213His allele in Asians was less than Caucasians.

Heterogeneity analysis
There was a significant heterogeneity in overall comparisons under all genetic models among the overall 20 studies of the SULT1A1 Arg213His (rs9282861) polymorphism (e.g., allele model: Q= 72.44 on 19 d.f (Q), p= ≤0.001, I 2 = 73.77%; and dominant model: Q= 189.01 on 17 d.f (Q), p= ≤0.001, I 2 = 91%). Therefore, we have performed subgroup analyses by ethnicity and HWE status to explore the source of heterogeneity. The I 2 decreased obviously and p value exceeded 0.05 under the homozygote, heterozygote and recessive models among Asians, while it was still significant under all genetic models among Caucasians ( Table 2). The results indicated that ethnicity may be a source of heterogeneity in the current meta-analysis.

Test of heterogeneity and sensitivity analyses
There was significant heterogeneity among these studies for SULT1A1 Arg213His (rs9282861) polymorphism in the allele mode (A vs. G: P<0.001), the homozygote model (AA vs. GG: P<0.001), the heterozygote model (AG vs. GG: P=0.004), the dominant model (AA+AG vs. GG: P<0.001), and the recessive model (AA vs. AG+GG: P=0.008). Then, the source of heterogeneity was assessed by meta-regression analysis. However, the ethnicity, genotyping methods, studies quality and source of population did not contribute to substantial heterogeneity in this meta-analysis (Table 2). In addition, the sensitivity analysis was performed to assess the reliability and conclusiveness of the overall results by repeating the meta-analysis sequentially removing each individual study. However, the results showed that the pooled ORs were not considerably affected by removing any individual study in all five genetic models, which indicated the reliability of our results.

Publication Bias
We have used both Begg's funnel plot and Egger's test to access the small study effects of the literatures. The visual inspection of funnel plot revealed no obvious asymmetry in the allele (Figure 2A), homozygote, heterozygote and recessive models. However, the results of Egger's test showed a publication bias for association of SULT1A1 Arg213His (rs9282861) polymorphism with risk of breast cancer in dominant model (AA+AG vs. GG: PBegg=0.047, PEgger=0.007, Figure 2B). Therefore, the Duval and Tweedie non-parametric ''trim and fill'' method was used to adjust for publication bias. Meta-analysis with and without ''trim and fill'' did not draw different conclusion, indicating that the results were statistically robust (Fig. 3B).

Figure 2:
Forest plot of the association between SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer disease. "A" represents allele model (A vs. G); and "B" represents the dominant model (AA+AG vs. GG). Every study was represented by a square whose size was proportional to the weight of the study. Diamond indicated summary odds ratios (OR) with its corresponding the pseudo 95% confidence limits (95% CI) Figure 3: Begg's funnel plots of publication bias test for the association between the SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer risk. "A" represents the allele model (A versus G); and "B" represents the dominant model (AA+AG vs. GG, with trim and fill test). The dots represent specific studies for the indicated association.

Discussion
The pathogenesis of breast cancer is complex and genetic factor play an important role in breast cancer susceptibility. An increasing number of publications on genetic association studies, genome-wide association studies (GWASs), and relate meta-analyses have been published to clarify the association between different gene polymorphisms and breast cancer. To our knowledge, this is the most comprehensive and robust meta-analysis to explore and report the association between the SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer. The pooled ORs of 20 case-control studies with 11,077 cases and 14,798 controls in the current meta-analysis concluded that there was a significant correlation between SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer under allele mode (A vs. G: OR=1.117, 95% CI: 1.011-1.233, P=0.029) and homozygote model (AA vs. GG: OR=1.288, 95% CI: 1.036, 1.601, P=0.022).
Our results are inconsistent with the previous metaanalysis by Xiao et al., which not reported a significant association between SULT1A1 Arg213His polymorphism and breast cancer risk 12  Between-study heterogeneity might distort the conclusion of a meta-analysis 43,44 . The study designs, source of controls subjects, ethnicity, genotyping method, sample size, lifestyle, subject's age and gender may contribute to the heterogeneity 45 . In the present meta-analysis, significant between-study heterogeneity was detected across studies under all models; therefore, we utilized the random-effects model to summarize the ORs. Additionally, in order to make the conclusion more credible, we performed the publication bias analysis and sensitivity analysis. Funnel plots suggested that no obvious publication bias was observed. Xiao et al. 12 removed the study conducted by Khvostova et al. 34 due to the low-quality and results indicated that this study influences the pooled estimates and the heterogeneity most in breast cancer subgroup. We have also removed Khvostova et al. 34 study, but there was no obvious effect on the pooled ORs and the corresponding CIs. In addition, we have performed the sensitivity analysis by removing those studies departed from HWE, but the results did not change meaningfully by excluding six studies departed from HWE or one study with low-quality. We suggest the relative small sample size status may be the reason for Xiao et al. 12 observations.
Despite the advantage of large sample size and no evidence of publication bias, the meta-analysis had several limitations that should be taken into account. First, the population data of SULT1A1 Arg213His (rs9282861) polymorphism are limited for the subgroup analysis, since there were not enough studies in other population or regions such as Africans and Middle East in this meta-analysis. Therefore there was insufficient statistical power to demonstrate the associations between the SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer risk in Africans or Middle East. Second, we only included articles that were published in English, and language bias might exist. Third, because of moderate to high heterogeneity in all five genetic models, we used the random effect model, which is not as reliable as the fixed-effects model and may potentially restrict the interpretation of the pooled risk estimates. Although the degree of heterogeneity was reduced (but not disappeared) by subgroup analyses based on ethnicity and HWE status, other sources of heterogeneity were not checked. Forth, in the current meta-analysis the design of all included studies was case control, therefore results more prone to selection bias. Fifth, the current meta-analysis results were based on unadjusted estimates, while a more precise analysis should be performed if all individual data available, which would allow for the adjustment by other co-variants including age, environmental exposures, smoking status, reproductive history, menopausal status and other lifestyle factors. Finally, gene-gene and gene-environment interactions which may modulate the breast cancer susceptibility were limited owing to the lack of the sufficient data in the eligible studies.
In summary, despite the limitations, this study is the most comprehensive, best available evidence and accurate metaanalysis focusing on the association between SULT1A1 Arg213His (rs9282861) polymorphism and risk of breast cancer. The present meta-analysis provided evidence of association between SULT1A1 Arg213His (rs9282861) polymorphism and increased breast cancer risk, especially among Asians. These results suggest that the SULT1A1 Arg213His (rs9282861) polymorphism is of high clinical relevance among Asians and would be a useful marker to identify women who are at higher risk for breast cancer. Moreover, well-designed epidemiological studies with larger sample size, carefully matching cases and control subjects and more ethnic groups are needed to validate these results. Further studies may focus on the influence of gene-gene and geneenvironment interactions on the association of SULT1A1 Arg213His (rs9282861) polymorphism and breast cancer.